The present invention relates generally to battery saver circuits and, in particular, to an improved battery saver circuit especially suited for use in personal paging receivers intended for sequential tone code operation. Such paging receivers produce a single alert signal in response to the reception of a predetermined code.
Battery saving circuits are used to minimize power consumption by periodically supplying power to a receiver in short bursts instead of continuously. Presently known circuits operating in radio receivers periodically supply power, search for the presence of an RF (radio frequency) carrier and then, if a carrier is found, extend the time that power is supplied to permit a further search for a predetermined sequential tone code. Such squelch operated battery savers have a significant disadvantage in that every receiver within the system is activated whenever any transmission of a carrier occurs, regardless of which individual paging receiver is intended to be selectively reached. If paging signals are continuously being broadcasted by a transmitter, this type of battery saving circuit will not save any power since all of the individual receivers will be on all of the time.
A previous partial solution has been to provide battery saving circuits which extend the time that power is applied to the receiver only after a first predetermined tone has been received. In this type of circuit, the first tone (preamble) must be of a sufficiently long duration such that the periodic supplying of power to the receiver will always result in a detection of this first tone, regardless of when this first tone begins. These circuits then proceed to look for the rest of a predetermined sequential code. Whenever the next proper sequential tone is not detected, the preamble tone is again searched for. A disadvantage of such a system, however, is that each individual message must be preceded by the preamble tone which is the first tone in each sequential code. When many messages are to be transmitted, this results in a substantial increase in the total amount of broadcast time. Consequently, the power drain on all receivers having the same first code (preamble) tone is increased and the total number of messages which can be broadcasted in any given time interval is severely limited. It is also possible for such prior battery saver circuits to mistakenly identify a subsequent tone as the first (preamble) tone of the code, and thus incorrectly apply power to a radio receiver.